HSDPA (High Speed Downlink Packet Access) is known as a communication method to carry out high-speed packet transmission through a downlink of a mobile communications system. Details of HSDPA are reported in 3GPP TR (Technical Report) 25. 858, V5.0.0 (March, 2002), and the overall disclosure of it is incorporated into this description by reference.
FIG. 18 is a block diagram showing a typical mobile communication system using HSDPA. The mobile communication system includes a base station 1, a mobile station 2, and a base station control apparatus 3 connected to the base station 1. The base station 1 and the mobile station 2 are connected through a radio link.
A downlink of the radio link connecting the base station 1 and the mobile station 2 is composed of CPICH (common pilot channel), HS-PDSCH (high speed physical downlink shared channel), HS-SCCH (high speed shared control channel), and downlink DPCH (downlink dedicated physical channel). CPICH is used to transmit a pilot signal to all mobile stations in a cell controlled by the base station 1. The pilot signal is transmitted from the base station 1 in a predetermined electric power, and is used for path search, estimation of a transmission path, and measurement of DL (Downlink) receiving quality. HS-PDSCH is used to transmit a user data as a packet. HS-PDSCH is used in a time divisional multiplexing manner, and is shared by the mobile stations 2. The HS-SCCH is used to transmit a control data necessary to receive a packet transmitted through the HS-PDSCH. Each of the mobile stations 2 receives the packet transmitted through the HS-PDSCH based on the control data. Each of the mobile stations 2 can receive four of HS-SCCH at maximum. HS-SCCH is used in a time divisional multiplexing manner, and is shared by the mobile stations 2. The downlink DPCH is used to transmit an individual channel data from the base station 1 to the mobile station 2. The individual channel data includes a user data, an upper layer control data and the like. The downlink DPCH is composed of a DPCCH (dedicated physical control channel) and DPDCH (dedicated physical data channel). DPCCH is used to transmit such a control data for a physical layer as TPC (transmit power control) bits, which are transmission power control data of DPDCH, and TFCI (transport format combination indication) that indicates a configuration of the DPDCH. DPDCH is used to actually transmit the individual channel data.
On the other hand, an uplink is composed of HS-DPCCH (high speed dedicated physical control channel) and uplink DPCH (uplink dedicated physical channel). HS-DPCCH is used to transmit CQI (channel quality indication) determined based on a result of CPICH quality measurement, and ACK/NACK message (acknowledgement/negative acknowledgement message). The ACK/NACK message is a data for notifying to the base station 1 that the mobile station 2 has correctly received a packet. The uplink DPCH is used to transmit the individual channel data to the base station 1 from the mobile station 2. The configuration of the uplink DPCH is the same as that of the downlink DPCH.
Transmission of packets from the base station 1 to the mobile station 2 in HSDPA is carried out roughly as described below. The mobile station 2 always monitors HS-SCCH (High Speed Shared Control Channel) of 4 channels at a maximum specified by the base station 1. The mobile station 2 determines whether a mobile station ID (identifier) transmitted through HS-SCCH coincides to a mobile station ID assigned to itself. When finding the mobile station ID assigned to itself in a control data transmitted through HS-SCCH, the mobile station 2 uses the control data transmitted through HS-SCCH to receive a packet transmitted through HS-PDSCH after the transmission of the mobile station Id by a predetermined delay time.
The base station 1 and the mobile station 2 immediately transmit individual channel data through the downlink DPCH or the uplink DPCH when the individual channel data must be transmitted.
One problem of HSDPA is in that power consumption of the mobile station is large. Two reasons are found in the large power consumption. One reason is in that the mobile station needs to always monitor HS-SCCH because a timing when the packet is transmitted is not informed. Therefore, the mobile station consumes a large amount of power even for reception of the packet for a short period of time. Especially, this problem is serious in HSDPA that is used for a service in which data download is intermittently repeated, as in web browsing. The other reason of the large power consumption is in that DPCH must be always set to an active state. When individual channel data needs to be transmitted, HSDPA is defined such that the individual channel data is transmitted through uplink/downlink DPCH. This means that it is necessary to set DPCH to the active state, regardless of presence or absence of packet transmission. More specifically, even when the mobile station does not transmit the individual channel data, the mobile station carries out on reception and transmission through DPCCH on the uplink/downlink DPCH such that the individual channel data can be immediately transmitted, if necessary. This undesirably increases the power consumption of the mobile station.
The undesirable increase in the power consumption of the mobile station also causes a problem in communication service other than HSDPA. For example, an individual channel must be always set to the active state regardless of presence or absence of data transmission, even in a packet transmission through the individual channel and control signal transmission for an upper layer such as a hand-over and an application. Thus, in these communication services, there is such a problem that the mobile station continues to consume power even when no data is transmitted.
Therefore, it is convenient if a mobile communication system is provided that immediately can transmit packet and/or control data, if necessary, while reducing power consumption of the mobile station.